With the object of clarifying the correlation between the structure and function of biopolymers, especially that of proteins and nucleic acids, researches by means of the fluorescent probes are widely conducted. For conducting the research works on the nucleic acids, methods generally practiced, including one in which a very small amount of fluorescent base existing in the nucleic acid is used as a probe and another in which a fluorescent molecule is introduced chemically into the nucleic acid to be used as a probe. As the examples of nucleoside which has a fluorescent base, there are fluorescent etheno derivatives, expressed by the following formulas, which are obtained by treating adenosine or cytidine with chloroacetaldehyde: ##STR5##
The ethenoadenosine has especially been used in various studies as the fluorescent probe because of its strong fluorescence intensity in neutral condition; unfortunately, it is not capable of forming the complementary base pairing.